Titanium aluminide based alloy

ABSTRACT

The invention concerns alloys made through the use of melting and powdered metallurgical techniques on the basis of titanium aluminides with an alloy composition of Ti-z Al-y Nb where 44.5 Atom % ≦z≦47 Atom %, 44.5 Atom % ≦z≦45.5 Atom %, and 5 Atom % ≦y≦10 Atom % with possibly the addition of B and/or C at a content between 0.05 Atom % and 0.8 Atom %. Said alloy is characterized in that it contains a molybdenum (Mo) content ranging between 0.1 Atom % to 3.0 Atom %.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 11/805,043, filed May 21, 2007, which is acontinuation of International Patent Application No. PCT/EP2005/009402filed on Sep. 1, 2005, which claims priority to German PatentApplication No. 10-2004-056582.1 filed on Nov. 23, 2004, subject matterof these patent documents is incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The invention concerns alloys made through the use of melting andpowdered metallurgical techniques on the basis of titanium aluminideswith an alloy composition of Ti-z Al-y Nb where 44.5 Atom % ≦z≦47 Atom%, especially where 44.5 Atom % ≦z≦45.5 Atom %, and 5 Atom % ≦y≦10 Atom% with possibly the addition of B and/or C at a content between 0.05Atom % and 0.8 Atom %.

BACKGROUND OF THE INVENTION

Titanium aluminide alloys have properties which make those alloys highlysuitable for use as light weight work materials, especially for hightemperature applications. For industrial practice those alloys are ofspecial interest which are based on an intermetalic phase γ-(TiAl) withtetragonal structure and along with the majority phase γ-(TiAl) alsocontain minority portions of the intermetalic phase α₂(Ti₃Al) withhexagonal structure. These γ-titanium aluminide alloys distinguishthemselves by properties such lightweight (3.85-4.2 g/cm³), high elasticmodulus, high strength and creep resistance up to 700° C., which makesthem attractive as work materials for moving parts at high operatingtemperatures. Examples of such uses are for turbine blades in aircraftengines and in stationary gas turbines, engine valves and hot gasventilators.

In the technically important area of alloys with aluminum contentbetween 45 Atom % and 49 Atom % there appears during the solidificationof the melt and during the subsequent cooling a series of phase changes.The solidification can take place either entirely by way of β-mixedcrystal cubic space centered structure (high temperature phase) or intwo peritectic reactions in which α-mixed crystals with hexagonalstructure and the γ-phase participate.

It is further known that the element niobium (Nb) leads to an increasein strength, creep resistance, oxidation resistance and also ductility.With the element boron which is practically insoluble in the γ-phase afine graining can be achieved both in the cast condition and also afterreshaping with subsequent heat treatment in the α-region. An increasedportion of the β-phase in the structure as a result of low aluminumcontent and high concentration of β-stabilizing elements can lead to acoarse dispersion of this phase and to an impairment of the mechanicalproperties.

The mechanical properties of γ-titanium aluminide alloys are, as totheir deformation and break behaviors, but also because of thestructural anisotropy of the preferred use of laminated structures orduplex-structures, strongly anisotropic. For a desired use of structureand texture in the making of components from titanium aluminides,casting methods, different powdered metal metallurgies and reshapingprocesses as well as combinations of these manufacturing methods areuseable.

From the publication of Y-W. Kim and D. M. Dimiduk in “StructuralIntermetallics 1997”, editors M. V. Nathanal, R. Darolia, C. T. Liu, P.L. Martin, D. B. Miracle, R. Wagner, M. Yamaguchi, TMS, Warrendale Pa.,1996, page 531 it is known that in the course of different developmentprograms the effect of a large number of alloying elements with respectto constitution, structural tuning in different manufacturing processesand individual properties have been investigated. The discoveredrelationships are thereby similarly complex as for the case with theother structured metals, for example, steels and can only be summarizedby rules which are limited and of very general form. Therefore certainmixtures can have exceptional combinations of properties.

A titanium aluminide alloy is known from EP1 015 605 B1 which has astructural and chemically homogenous structure. In this case themajority phases γ(TiAl) and α₂ (Ti₃Al) are separated into a finedispersion. The disclosed titanium aluminide alloy with an aluminumcontent of 45 Atom % distinguishes itself by exceptionally goodmechanical properties and high temperature properties.

A general problem of all titanium aluminide alloys is their lowductility. For a long time one has not succeeded in improving thepregiven high brittleness and low damage tolerance of titanium aluminidealloys arising from the nature of the intermetallic phases (compare“Structural Intermetallics 1997”, page 531, see above). For many of theabove mentioned uses indeed plastic fracture elongations of ≧1% aresufficient. For the making of turbines and motors however it isnecessary that this minimum amount of ductility be guaranteed inindustrial manufacturing throughout large batch numbers. Since theductility is sensitively dependent on structure in industrialmanufacturing processes it is extremely difficult to assuredly obtain ahighly homogenous structural configuration. For high tensile strengthalloys a maximum tolerable defect size, for example the maximum grain orlamina colony size, is very small so that for such alloys a very highstructural homogeneity is desirable. This homogeneity can however,because of the unavoidable fluctuation of the alloying mixture from, forexample ±0.5 Atom % in aluminum content, only be reached withdifficulty.

At the present time of the many possible structural types of γ-titaniumaluminide alloys only lamellar and so called duplex structures are takeninto consideration for high temperature uses. Upon the cooling from thesingle phase region the α-mixed crystals first appear while plates ofthe γ-phase crystallographically become oriented and separate from theα-mixed crystals.

Compared to this, duplex structures consisting of lamina colonies andγ-grains arise when the material has been heated into the second phasearea α+γ. Then upon cooling the α-grains lying in the second phase areaagain change into two phased lamina colonies. Above all, coarsestructural components exist in γ-titanium aluminide alloys since duringthe running through of the α-area large α-grains are formed. This canindeed happen during the solidification when large stalk crystals of theα-phase are formed from the melt. Accordingly as much as possible thesingle phase area of the α-mixed crystals must be avoided duringprocessing. Since in practice however fluctuations in the compositionand processing temperatures appear and thereby locally vary theconstitution in work pieces, the formation of large lamina colonies isnot to be prevented.

Proceeding from this state of the art the present invention has as itsobject the making available of a titanium aluminide alloy with a fineand homogeneous structural morphology, as to which alloy the variationsof the alloy composition as well as unavoidable temperature fluctuationswhich appear during manufacturing processes of industrial practice havehardly any or no significant effect on the homogeneity of the alloy, andespecially without having to make any basic changes in the manufacturingprocesses. Therefore a further object of the invention is to makeavailable a structural component consisting of a homogenous alloy.

SUMMARY OF THE INVENTION

This object is solved by means of an alloy based on titanium aluminidemade through the use of melting and powdered metallurgical technologieswith an alloy composition of Ti-z Al-y Nb where 44.5 Atom % ≦z≦47 Atom%, especially where 44.5 Atom %≦z≦45.5 Atom %, and 5 Atom % ≦y≦10 Atom%, which is further formed in that this alloy contains molybdenum (Mo)in the range of between 0.1 Atom % to 3.0 Atom %. The remainder of thealloy is made up of Ti (titanium).

Investigations have shown that the alloying of molybdenum with titaniumaluminide having a niobium portion usually results in an alloy for whichthe β-phase is not stable over the entire temperature region, andtherefore in a customary process procedure such as extrusion theremainder of the high temperature β-phase dissolves, and a betterstructural homogeneity of the alloy is obtained. In this way over theentire temperature range relevant to the before mentioned manufacturingprocess a portion of the volume of the β-phase without grain coarsenessis realized. This type of alloy according to the invention therefore,because of the fine and very uniform dispersion of the β-phase, has ahomogenous structure with high strength values.

Therefore an alloy is presented by the invention which is suitable as alightweight work material for high temperature applications, such as forturbines blades or engine and turbine components. The alloy of theinvention is made through the use of casting metallurgy, meltingmetallurgy or powdered metal metallurgy methods or by the use of thesemethods in combination with reshaping techniques.

Above all in the case of Ti-(44.5 Atom % to 45.5 Atom %) Al-(5 Atom % to10 Atom %) Nb the addition of molybdenum at a content of about 1.0 Atom% to 3.0 Atom % leads to good microstructures with a high structuralhomogeneity.

Moreover an alloy according to the invention has a composition of Ti-zAl-y Nb-x B where 44.5 Atom % ≦z≦47 Atom %, especially where 44.5 Atom %≦z≦45.5 Atom %, 5 Atom % ≦y≦10 Atom % and 0.05 Atom % ≦x≦0.8 Atom %, ora composition of Ti-z Al-y Nb-w C where 44.5 Atom % ≦z≦47 Atom %,especially where 44.5 Atom % ≦z≦45.5 Atom %, 5 Atom % is ≦y≦10 Atom %and 0.05 Atom % ≦w≦0.8 Atom %, each of which alloys contains molybdenum(Mo) in the region of between 0.1 Atom % to 3 Atom %, especially in theregion of between 0.5 Atom % to 3 Atom %.

Alternatively the alloy is made up of Ti-z Al-y Nb-x B-w C where 44.5Atom % ≦z≦47 Atom %, especially where 44.5 Atom % ≦z≦45.5 Atom %, 5 Atom% ≦y≦10 Atom %, 0.05 Atom % ≦x≦0.8 Atom % and 0.05 Atom % ≦w≦0.8 Atom %and additionally of molybdenum in the region of between 0.1 Atom % to 3Atom %.

By means of the given alloying and the corresponding alloyingproportions high strength γ-titanimium aluminide alloys with a finedispersion of the β-phase are created for a wide range of processingtemperature.

In the case of the present invention the strived for structuralstability and process security are thereby achieved in that theappearance of single phase regions are avoided over the entiretemperature region traversed in the manufacturing processes and uponuse, by the aimed for inclusion of the cubic space centered β-phase.Principally the β-phase appears as the high temperature phase for alltechnical titanium aluminide alloys at temperatures ≧1350° C.

From the literature it is known that this phase can be stabilized at lowtemperatures by different elements such as Mo, W, Nb, Cr, Mn, and V. Thespecial problem with the alloying of these elements exists however inthat the β-stabilizing elements have to be very accurately tuned to theAl content. Moreover in the case of the addition of these elementsundesirable exchange effects appear which lead to higher portions of theβ-phase and to a coarse dispersion of this phase. Such a constitution ismost disadvantageous for the mechanical properties. Further, theproperties of the β-phase are dependent on the alloying elements andtheir composition. Especially the constitution must be so chosen so thata precipitation of the brittle ω-phase from the β-phase must besubstantially avoided. Because of this relationship an alloyingcomposition is presented whereby for the mechanical properties anoptimum composition and dispersion of the β-phase can be realized for awide region of processing temperatures. At the same time the bestpossible strength properties are achieved.

According to a preferred form of the invention the alloy likewisecontains boron, preferably with a boron content in the alloy in the areaof from 0.05 Atom % to 0.8 Atom %. The addition of boron leadsadvantageously to the formation of stable precipitates which likewisecontribute to the mechanical hardening of the alloy and to thestabilization of the structure.

The object of the invention is further solved by a constructioncomponent made from an alloy of the invention. To avoid repetitionreference is made to the previous exposition.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention, without limitation to the generalthought of the invention, is described by way of exemplary embodimentswith reference to the accompanying schematic drawings, to which inregard to publication reference is made for all details of the inventionnot more closely explained in the text. The drawings are:

FIG. 1 shows a raster electron microscope picture of a cast block havingan alloying of Ti-45Al-8Nb-0.2C (Atom %);

FIGS. 2 a to 2 c each shows a picture of the structure in an alloy ofTi-45Al-8Nb-0.2C (Atom %) taken by a raster electron microscope afterdifferent processing steps;

FIGS. 3 a and 3 b each shows a picture of the structure in an alloy ofthe invention of Ti-45Al-Nb-2 Mo (Atom %) after different processingsteps, and

FIG. 4 is a diagram with tension-elongation curves resulting from testsof the alloy Ti-45Al-5Nb-2 Mo (Atom %).

DETAILED DESCRIPTION

FIG. 1 shows two pictures of a structure in a cast block made of thealloy Ti-45Al-8Nb-0.2 C (Atom %). The pictures as well as the furtherpictures in the following figures were taken by means of back scatteredelectrons in a raster electron microscope.

The structure (FIG. 1) shows lamina colonies of the a₂-phase and theγ-phase, which originate from former γ-lamina. The former γ-lamina areseparated by stripes of bright pictured grains of β-phase or B2-phase.The α-lamina next formed in the β- α-conversion decay upon furthercooling into α₂-lamina and γ-lamina.

In FIGS. 2 a to 2 c two further pictures of the structure of the alloyTi-45Al-8Nb-0.2C taken in the raster electron microscope and afterdifferent processing steps are shown. FIG. 2 a shows the structure afterextrusion at 1230° C. The extrusion direction runs horizontally. Thestructure shows grains of the α₂- and β-phase, with the cubic spacecentered β-phase having vanished.

FIG. 2 b shows the structure of the alloy after the extrusion at 1230°C. and a further forging step at 1100° C. This structure shows grains ofthe α₂- and γ-phase and a few α₂/γ lamina colonies.

In FIG. 2 c is shown the structure of the alloy after extrusion at 1230°C. and a subsequent heat processing at 1330° C. This structure exhibitslikewise grains of the α₂- and γ-phase. The picture shows a fullylaminar structure with lamina of the α₂- and γ-phase. The lamina colonysize has a value of about 200 μm, with colonies also appearing which areclearly larger than 200 μm.

As in the structure illustrated in FIG. 2 a, also in the structuresillustrated in FIGS. 2 b and 2 c the cubic space centered phase does notappear. So the β-phase in this temperature range with a heat processingafter the extrusion is thermodynamically not stable.

In FIGS. 3 a and 3 b are illustrated raster electron microscope picturesof the structure of an alloy in accordance with the invention.Proceeding from an alloy of Ti-45Al-5Nb the alloying agent molybdenumwas added at 2 Atom %. This starting alloy Ti-45Al-5Nb-2Mo is based on acomposition as described in European Patent EP 1 015 650 B1.

FIGS. 3 a and 3 b show the structure of this alloy of the inventionafter an extrusion at 1250° C. and a subsequent heat treatment at 1030°C. (FIG. 3 a) as well as observed at 1270° C. (FIG. 3 b).

The structure of FIG. 3 a exhibits grains of the α₂-phase, the γ-phaseand the brightly pictured β-phase, with the latter being arranged instrips. The structure in FIG. 3 b shows lamina colonies of α₂-andγ-phases as well as grains of the brightly pictured β-phase, which againhave precipitated from the γ-phase.

The structures of FIGS. 3 a and 3 b are fine, very homogenous and showuniform distribution of the β-phase. After the heat treatment of 1030°C. a globular structure is presented, with it having grains of β-phasein strips parallel to the extrusion direction, while the material heattreated at 1270° C. exhibits a very homogenous, fully lamellar structurewith uniformly distributed β-grains (FIG. 3 b).

The colony size of the alloy Ti-45Al-5Nb-2Mo has a value of between 20to 30 μm and is therefore at least about 5 times smaller than in thefully laminar structure of γ-titanium aluminide alloy. Moreover, in theβ-phase the γ-phase has been eliminated so that the β-grains are veryfinely subdivided. Therefore, in summary, a very fine and homogenousstructure has been achieved.

Tests have shown that this fine and homogenous structure morphologyafter heat treatment is present for the entire high temperature range upto 1320° C. The structures show clearly that over the entire temperaturerange relevant for the manufacturing processes a sufficient volume ofthe β-phase is provided and the grain growth is effectively suppressed.

In tension tests carried out on the material which was heat treated at1030° C., at room temperature a stretch limit of 867 MPa, a tensilestrength of 816 MPa and a plastic elongation at rupture at 1.8% weremeasured.

FIG. 4 shows measured tension-elongation curves from test of the alloyTi-45Al-5Nb-2Mo in tension tests. The test material was extruded at1250° C. and subsequently subjected to a heat treatment for two hours at1030° C. and was then subjected to an oven cooling. The curves taken at700° C. and 900° C. show that the alloy is suitable for many hightemperature applications. By the alloying of a small amount ofmolybdenum a very uniform microstructure in the alloy is achieved sothat this alloy can be well used as a high temperature work material.

Moreover in FIG. 4 the results of a tension test at room temperature(25° C.) on the material of the invention is illustrated, with thetension σ in MPa being shown against the elongation ε in %. Thereby anelongation limit increase was found which otherwise up to now has notbeen observed for γ-titanium aluminide alloys. This represents anindication of an especially fine and homogenous structure. Theelongation limit increase indicates that the material can react to localtensions by plastic flow, which is very beneficial for ductility anddamage resistance.

The homogeneity of the alloy of the invention in the region of relevantprocessing temperatures is not dependent on technically unavoidablefluctuations of the temperature or of the composition.

The titanium aluminide alloys of the invention are made through the useof metallurgical casting or powdered metal techniques. For example, thealloys of the invention can be processed by hot forging, hot pressingand hot extrusion and hot rolling.

The invention offers the advantage that despite the fluctuations of thealloying composition appearing with the industrial finishing andunavoidable processing requirements as previously, a titanium aluminidealloy with very uniform microstructure and high strength has been madeavailable.

The titanium aluminide alloy of the invention achieve high strength upto a temperature in the region of 700° C. to 800° C. as well as goodroom temperature ductility. Therefore the alloys are suitable fornumerous areas of application and can for example be used for highlyloaded components or as titanium aluminide alloys for exceptionally hightemperatures.

1. An alloy made on the basis of titanium aluminide through the use ofmelting and powdered metallurgical techniques the alloy comprising Ti-zAl-y Nb where 44.5 Atom % is ≦z≦47 Atom %, and where 5 Atom % is ≦y≦10Atom %, and the alloy containing molybdenum (Mo) in between 0.1 Atom %to 3 Atom % and defining a β-phase present up to a temperature of about1,320 degrees C.
 2. An alloy as defined by claim 1 wherein 44.5 Atom %is ≦z≦45.5 Atom %.
 3. An alloy on the basis of titanium aluminide madewith the use of melting and powdered metallurgical techniques the alloycomprising Ti-z Al-y Nb-x B where 44.5 Atom % is ≦z≦47 Atom % and where5 Atom % is ≦y≦10 Atom % and 0.05 Atom % is ≦x≦0.8 Atom % and whereinthe alloy contains molybdenum (Mo) between 0.1 Atom % to 3 Atom % anddefining a β-phase present up to a temperature of about 1,320 degrees C.4. An alloy as defined by claim 3 wherein 44.5 Atom % is ≦z≦45.5 Atom %.5. An alloy on the basis of titanium aluminide made with the use ofmelting and powdered metallurgical techniques the alloy comprising Ti-zAl-y Nb-w C where 44.5 Atom % is ≦z≦47 Atom %, and where 5 Atom % is≦y≦10 Atom % and 0.05 Atom % is ≦w≦0.8 Atom %, and wherein the alloycontains molybdenum (Mo) between 0.1 Atom % to 3 Atom % and defining aβ-phase present up to a temperature of about 1,320 degrees C.
 6. Analloy as defined by claim 5 wherein 44.5 Atom % is ≦z≦45.5 Atom %.
 7. Analloy as defined by claim 5 wherein the alloy contains molybdenumbetween 0.5 Atom % to 3 Atom %.
 8. An alloy on the basis of titaniumaluminide made with the use of melting and powdered metallurgicaltechniques the alloy comprising Ti-z Al-y Nb-x B-w C where 44.5 Atom %is ≦z≦47 Atom %, and where 5 Atom % is ≦y≦10 Atom % and 0.05 Atom % is≦x≦0.8 Atom % and 0.05 Atom % is ≦w≦0.8 Atom %, and wherein the alloycontains molybdenum (Mo) between 0.1 Atom % to 3 Atom % and defining aβ-phase present up to a temperature of about 1,320 degrees C.
 9. Analloy as defined by claim 8 wherein 44.5 Atom % ≦z≦45.5 Atom %.
 10. Aconstruction component made from an alloy according to claim
 1. 11. Aconstruction component made from an alloy according to claim
 3. 12. Aconstruction component made from an alloy according to claim
 5. 13. Aconstruction component made from an alloy according to claim 8.